I assume you use dxVoodoo to enable GP3 to run on modern systems?
I don't know exactly how to change it properly, but in the graphics menu, there's the ADVANCED tab, and if there's a way to tick the "High Resolution Graphics (Hardware 3D)" - it's called something similar to that, but there's a "
Hardware 3D)" in it - if you can tick that, it's currently in Hardware mode. To change to Software mode, select another graphics card adapter in the menu.